This invention relates to a method and an apparatus for controlling heating and cooling systems for buildings. Thermostats are typically used to maintain space temperatures in buildings by controlling heating and cooling systems. This invention is more specifically related to such thermostats.
Known thermostats turn the heating and cooling system on or off whenever the space temperature deviates from the thermostat set-point temperature. When the set-point temperature is not met, the thermostat turns the heating and cooling system on and when the set-point temperature is met, it turns the system off. The limits of deviation from the set-point temperature (when the system is turned on and when the system is turned off) are known as the deadband of the thermostat. Current thermostats operate with a constant deadband, which is usually narrow. Since the single-speed heating and cooling system is normally designed for peak heating/cooling loads, it cycles on and off more frequently, when the outdoor temperatures are mild. This increased cycling decreases the system performance. Therefore, the present invention relates to a method and an apparatus that uses a variable deadband rather than a fixed deadband to decrease cycling of the single-speed heating and cooling system and to increase its energy efficiency without sacrificing comfort.
Residential heating and cooling systems operate at fixed speeds. Since the weather variation poses varying load on the building and the capacity of the heating and cooling system is constant, the system cycles to meet the building load. It is well known that cycling of heating and cooling systems results in as much as 40% of energy waste depending upon the climate and amount of over-sizing of the systems. A perfect solution to this problem is to utilize continuously variable speed heating and cooling systems that vary the capacity to meet the varying load. There have been attempts to introduce continuously variable speed systems into the residential market. However, these attempts have not been successful due to high cost and complexity of the variable speed systems.
Rather than attempting to eliminate cycling completely with expensive variable speed HVAC systems, the present invention reduces cycling in an inexpensive and practical way. The present invention achieves reduced cycling by implementing simple strategies in the existing thermostat technologies. The present invention is applicable to both electronic (programmable) and electromechanical thermostats.
A primary object of the invention is to provide a method which reduces cycling of heating and cooling systems used to condition buildings. Another object of the invention is to implement such a reduced cycling method into an electronic thermostat. Another object of the invention is to implement such a reduced cycling method into an electromechanical thermostat.
The present invention uses a variable deadband rather than a fixed deadband for the thermostat to control any space heating and cooling system. The deadband is kept constant and narrow during peak heating/cooling periods. The deadband is made wider when the outdoor temperatures are milder. Also, when the thermostat is set back during night or during periods of non-occupancy, the deadband is set wider. By increasing the deadband, the system cycling is reduced and its efficiency is increased.
It is to be clearly understood that the term xe2x80x9cwider deadbandxe2x80x9d is used herein to signify a wider deadband vis-a-vis a normal, relatively narrow deadband. Therefore, in its simplest form, the instant invention may utilize a dual deadband, for example in the case where the xe2x80x9cwider deadband is factory-set and fixed. However, the xe2x80x9cwider deadbandxe2x80x9d may be variable according to certain preferred embodiments, i.e. the width of the xe2x80x9cwider deadbandxe2x80x9d may be adjustable, to provide a xe2x80x9cvariable deadbandxe2x80x9d in order to obtain further adjustability and control over the cycling times of the HVAC system. The width of the xe2x80x9cvariable deadbandxe2x80x9d may be varied, for example, via direct input from a user, or alternatively as a function of time and/or temperature.
According to certain preferred embodiments, a separate fixed xe2x80x9cnormalxe2x80x9d deadband may be eliminated entirely where the width of the variable deadband is itself variable. In this embodiment, the variable deadband may be adjusted continuously or step-wise, for example as a function of a time schedule and/or the outdoor temperature. Such an embodiment allows greater adjustability and more precise control than, for example, a variable deadband thermostat with only a dual deadband.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.